Electrode for tissue stimulation

ABSTRACT

In an implantable electrode for an electrode lead for a stimulation device for stimulating tissue, the electrode is formed as a biocompatible piezoelectric electrode which is adapted to be in direct electrical contact with tissue for electrically and mechanically stimulating the tissue and for detecting electrical and mechanical evoked response of the stimulated tissue. The stimulation device can include circuitry for making a diagnosis of a heart condition using signals received from the implantable electrode.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates generally to an electrode for implantablestimulation devices such as heart pacemakers or defibrillators. Theinvention relates further to implantable leads and stimulation devicessuch as heart pacemakers or defibrillators which employ such anelectrode. Moreover, the invention relates to the use of the electrodefor diagnosing the condition of stimulated tissue. In particular, theinvention relates to an electrode which is adapted to electrically andmechanically transfer stimulation energy to tissue, to an electrodewhich is adapted to electrically and mechanically receive electrical andmechanical evoked response of the tissue to which stimulation energy hasbeen transferred and to an electrode which is adapted for a combinationof the two.

2. Description of the Prior Art

The life span of most pacemakers is dictated by the rate at which theirbatteries drain. Thus, a substantial effort has been directed towardminimizing the amount of energy used by pacemakers, while ensuring thatthe devices continue to deliver effective therapy. For example, demandpacemakers effectively reduce the battery drain by delivering pacingpulses only when required, i.e. if the pacemaker has not detected anyspontaneous activity. Another way to reduce the current consumption isto minimize the amplitude and/or the duration of the stimulation pulseto a value just above the threshold. However, there are for exampletimes when the heart emits an electrical signal, without providing acorresponding mechanical contraction (electromechanical dissociation).However, the pacemaker detects and interprets the electrical signal asan intrinsic beat or an evoked response. There are also times when theheart does not respond normally with increased cardiac output forincreased stimulation rate as for example for patients with coronaryartery disease during angina pectoris.

A way of minimizing the amount of energy needed for defibrillation,while ensuring that the defibrillators continue to deliver effectivetherapy, is disclosed in U.S. Pat. No. 5,433,731 a defibrillator havingmeans for supplying the heart with a mechanical shock instead of anelectrical shock. One embodiment discloses an electrode for supplying adefibrillation pulse, whereby the electrode is provided with an elementon its distal exterior, which presses against the heart tissue andconverts the electrical energy into mechanical energy. The element canfor example be a piezoelectric element.

U.S. Pat. No. 5,304,208 discloses a cardiostimulator device having anelectrode including an acceleration sensor for detecting theacceleration to which the cardiac mass is subjected as a reaction to anycontraction whatsoever of the cardiac mass. The acceleration sensor issolely sensitive to inertial forces and can therefore be located in anentirely rigid capsule and consequently be entirely insensitive to thepressure in the ventricle or the atrium, and to pressure which thecardiac wall can exert, particularly on the distal electrode.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an implantableelectrode, an electrode lead embodying an implantable electrode, and animplantable stimulation device employing an electrode lead, which avoidthe aforementioned disadvantages of known leads and stimulators.

This object is inventively achieved in accordance with the principles ofthe present invention in an implantable electrode for an implantablestimulation device, the implantable electrode having a conductive corecovered by a biocompatible piezoelectric material adapted to be indirect contact with tissue when implanted, and wherein the entirepiezoelectric surface of the implantable electrode is adapted forelectrically and mechanically transferring stimulation energy to thesurrounding tissue, and/or is adapted to electrically and mechanicallyreceive electrical and mechanical evoked or intrinsic responses oftissue, to which stimulation energy can be transferred.

The above object is also achieved in an electrode lead having animplantable electrode as described above, and further having a conductorfor delivering stimulation energy to the piezoelectric material and/orfor conducting signals from the piezoelectric material, the signals fromthe piezoelectric material representing the aforementioned electricaland mechanical evoked or intrinsic response of the tissue.

The above object is also achieved in an implantable stimulation devicefor stimulating tissue having an electrode lead with an implantableelectrode as described above, and further having a stimulation pulsegenerator for delivering stimulation energy to the implantableelectrode, and a detector for receiving signals from the implantableelectrode corresponding to the aforementioned electrical and mechanicalevoked or intrinsic response of the tissue.

An advantage of the invention is that it is possible to more reliablystimulate heart tissue and to detect a heart contraction. As a result alower energy consumption is ensured.

According to the invention, the piezoelectric electrode is formed of abiocompatible piezoelectric material adapted to be in direct contactwith the tissue, it surprisingly has been found that the conductivelayer hitherto believed necessary can be excluded. In one embodiment thepiezoelectric electrode is the tip electrode and in another embodimentthe piezoelectric electrode is the ring electrode. In a preferredembodiment the stimulation pulse generator supplies the electrode with achopped stimulation pulse.

DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are respective schematic illustrations of twoembodiments of a top electrode in accordance with the invention, forelectrically and mechanically stimulating tissue and detecting an evokedresponse.

FIG. 1C is a schematic illustration of an embodiment of a ring electrodein accordance with the invention, for electrically and mechanicallystimulating tissue and for detecting an evoked response.

FIG. 1D is a schematic illustration of an electrode lead in accordancewith an embodiment of the invention.

FIG. 1E is a schematic equivalent circuit of the piezoelectric electrodein accordance with an embodiment of the invention.

FIG. 2 is a pulse diagram of the detector input signal generated by theelectrode in accordance with an embodiment of the invention, andincluding the stimulation pulse, the electrical evoked response, and themechanical evoked response.

FIG. 3 is a schematic illustration of a cardiac pacemaker having anelectrode in accordance with the invention.

FIG. 4 is a schematic circuit diagram of a cardiac pacemaker inaccordance with a first embodiment of the invention.

FIG. 5 is a schematic circuit diagram of a cardiac pacemaker inaccordance with a further embodiment of the invention.

FIG. 6 is a schematic drawing of single lead wherein a piezoelectricelectrode in accordance with an embodiment of the invention is placed atthe tip of the lead and an intravascular defibrillation electrode isplaced behind the piezoelectric electrode.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 1A and 1B, show an electrode 10 for a heart pacemaker 100 (seeFIG. 3). The electrode 10 includes a conductor 20 enclosed by aninsulator 30, e.g. silicon rubber. The conductor 20 is at one end incontact with an electrically conductive core 40, which is covered with apiezoelectric material 50. So as to obtain a high capacitance usually ofthe order 10-100 nF, the layer of piezoelectric material is very thin(0.1-5 μm). The piezoelectric material 50 is biocompatible. The metalcore 40 and the piezoelectric layer 50, i.e. the piezoelectricelectrode, form the tip of the electrode lead 11. FIGS. 1A and 1B show ahemispherical and a planar embodiment of the tip respectively, theplanar embodiment being more sensitive to how it is placed with respectto the myocardial tissue. In a preferred embodiment the conductor 20 ismade of the commonly used alloy MP35 and the conductive core 40 of e.g.graphite, titanium, platinum or iridium. The size of the electrode isabout the same as for standard electrodes and may for instance varybetween 1-10 mm².

It should be noted that the term biocompatible should be read asencompassing all materials that may be in direct contact with the tissuewithout adverse effects. Thus, the piezoelectric material PZT, amaterial that at least in some compositions contains lead and thereforesometimes is considered to be not biocompatible, possibly could betermed biocompatible when used in minute amounts. The amounts that wouldbe used in the above preferred embodiment could be termed minute in viewof the maximal amounts of lead that in the worst case could be releasedfrom the piezoelectric layer.

According to yet another embodiment of the invention, FIGS. 1C and 1Dshow a coaxial stimulating and sensing piezoelectric electrode 40, 50.The coaxial piezoelectric electrode 40, 50 is positioned about 1 to 15cm behind the tip having an endocardial stimulation electrode 200. Thisembodiment may e.g. be used in a single lead DDD pacemaker system asdisclosed in U.S. Pat. No. 5,476,499. The tip is thereby screwed intothe atrial myocardium and a loop descends into the ventricle and makescontact with the ventricular wall. The design of the lead 11 is suchthat the ring 10 of the lead 11 is found in the contact area and thering 10 comprises the coaxial piezoelectric electrode 40, 50. The lead11 must have two conductors in this case. One conductor 12 is connectedto the tip and atrial part of the DDD pacemaker. The other conductor 13is connected to the piezoelectric electrode 40, 50 and the ventricularcircuits of the pacemaker. The block schematics in FIGS. 3, 4 and 5 arethus still valid. The interactions between the atrial and ventricularparts of the DDD pacemaker are well known to the person skilled in theart of pacemakers.

FIG. 1E shows a schematic equivalent circuit 45 of the electrode inaccordance with an embodiment of the invention, whereby thepiezoelectric electrode 40, 50 comprises a voltage source V_(p) and acapacitor C_(p). The electrode 10 is further characterised by the tipsurface 50. The conductor 13, 20 electrically connects the electrode tothe electronics of the pacemaker.

A stimulation pulse delivered to the electrode 10 and thus to the piezoelectrode 45, will change the thickness of the piezoelectric materialduring the pulse and two pressure waves will be emitted therefrom, therebeing one pressure wave for each slope of the stimulation wave. Thecapacitor C_(p) of the piezoelectric electrode 40, 50 transmits theelectrical stimulation pulse to the heart cells.

To avoid charging of the piezoelectric material, the material can bedoped or contaminated with a conducting material such as carbon. It isconceivable to have different time constants for the charging. A shorttime constant, for instance 10-100 ms, entails that the charge has beendissipated before the mechanical response arrives. In this case, onlyfast events can be monitored/detected (>20 Hz). An alternative is toprovide a slow discharge during for instance 1-10 seconds, whichprevents a cumulative charge, but which permits a relatively low cut-offfrequency fg. If the total resistance against leaking over thepiezo-matenal is termed R and the total capacitance is termed R, thefollowing examples can be given:

Example 1, fast tip.

C=10 nF, R=500 kohm=>τ=RC=5 ms=>fg=32 Hz

Example 2, slow tip.

C=100 nF, R=500 Mohm=>τ=RC=0.5 s=>fg=0.32 Hz

FIG. 2 shows a pulse diagram of the detector input signal generated bythe electrode in accordance with an embodiment of the invention andillustrating the stimulation pulse Stim, the electrical evoked responseA and the electrical signal B corresponding to the mechanical evokedresponse. Consequently, a successful heart stimulation will be sensed astwo electrical signals by the detector 110 shown in FIG. 3. First themuscle cells close to the electrode will immediately after thestimulation pulse generate an electrical signal A related to the triggedion transport. Then the global heart muscle contraction will exert amechanical pressure on the piezo electrode 45 which generates the secondelectrical signal B. The electrical signal B arrives within a timewindow C after a certain time D of the electrical signal A. The timeinterval D depends on the location of the electrode and on the activityof the autonomic nervous system. However, the time interval D issubstantially constant for each individual. The time interval D isapproximately 5 to 100 ms if the electrode is located in the ventricle.Furthermore, the electrical signal B appears in a relatively narrow timewindow C, which is approximately 50 ms if the electrode is located inthe ventricle.

A control unit 130, e.g. a microprocessor, includes known means foranalyzing the detected electric signals A and B and how they relate toeach other and to the stimulation pulse, so that information regardingthe condition of the heart can be obtained. This information cantherefore be used as a diagnostic tool for analyzing the condition ofthe heart.

The control unit 130 may obtain information from the dual sensingdetector for analyzing the evoked response signals. It is e.g. oftendifficult to handle fusion beats in pacemakers comprising an autocapturefunction. A fusion beat is a cardiac depolarization (atrial orventricular) resulting from two foci. In pacing it typically refers tothe ECG waveform which results when an intrinsic depolarization and apacemaker output pulse occur simultaneously and both contribute to theelectrical activation of that chamber. Another difficulty when analyzingevoked response signals is related to the declining electrodepolarization after the stimulation pulse. If the polarization artefactis large, compared to the electrical signal generated by the heart, thecontrol unit 130 may interpret the polarization as a capture. A captureoccurs when the stimulation results in a heart contraction.

Using this electrode, a new possibility for the control unit 130 toverify capture has been created. If the electrical signal B does notfall within the time interval C, the heart contraction is probably notrelated to the stimulation pulse. If the electrical signal B arrivesbefore the time window C, a fusion beat is present, or the QRS detectorsensitivity is set too low, so that the pacemaker does not inhibit thepacing pulse. If the electrical signal B arrives after the time windowC, there is a loss of capture followed by a spontaneously released heartbeat.

If only the electrical signal A is present, the detector either sensesthe polarization artefact due to the sensitivity being too high andshould be adjusted, i.e. evoked response oversensing, or the patient hasa beat with electromechanical dissociation.

By analyzing the morphology, i.e. duration and amplitude, of theelectric signal B, information regarding the heart contractility can beobtained. For patients with coronary artery disease during anginapectoris, the contractile behaviour is changed. With the electrodeaccording to the invention it is possible for the pacemaker to detectthis adverse situation and start therapy. The pacing rate should bereduced until the attack is over. This function is especially importantfor physiologically rate controlled pacemakers such as the ones beingcontrolled by the venous oxygen contents.

Certain patients have a prolonged or varying time between the atrialstimulation A and the atrial evoked electrical response. By letting thecontrol unit 130 start the A-V timer in a two chamber pacing systemafter the detection of the electrical signal B corresponding to themechanical evoked response, instead of after the evoked electricalresponse, these patients will obtain a more stable heart function. TheA-V timer is the timer keeping track of the time elapsed between theatrial stimulation A and the ventricular stimulation V.

There are times when the heart in response to a stimulation pulse emitsan electrical signal, but does not actually contract (electromechanicaldissociation). However, the pacemaker detects and interprets theelectrical signal as an evoked response. Since the electrode accordingto the invention registers both electrical and mechanical evokedresponse, it can distinguish e.g. hemodynamically stable tachycardias atexercise from a pathological situation. Consequently, the electrodeaccording to the invention is suitable for therapy when using animplantable cardiac defibrillator.

FIG. 3 shows the schematic drawing of a heart pacemaker 100 for tissuestimulation. The heart pacemaker 100 contains a stimulation pulsegenerator 120 that has its output side connected via a lead 11 to anelectrode 10 applied in the ventricle of the heart for deliveringstimulation pulses to the heart. Of course, even though FIG. 3 shows theelectrode 10 to be located in the ventricle, the invention also coversthe electrode 10 being located in the atrium. The stimulation pulsegenerator 120 can be activated to deliver a stimulation pulse via acontrol line, which is connected to a corresponding output of a controlunit 130, e.g. a microprocessor. The stimulation pulse generated by thestimulation pulse generator 120 may be anyone of the stimulation pulsesknown to the skilled person. The duration of the each stimulation pulseas well as the amplitude thereof are set by the control unit 130. In theillustrated preferred embodiment, the control unit 130 has access to amemory 140 wherein a program that execute all functions of the heartpacemaker 100 via the control unit 130 is stored. The pacemaker 100 alsocontains a telemetry unit 150 connected to the control unit 130 forprogramming and for monitoring the functions of the pacemaker 100 and ofparameters acquired therewith on the basis of data exchange with anexternal programming and monitoring device (not shown).

In order to be able to acquire the reaction of the heart given astimulation, the pacemaker 100 also contains a detector unit 110 whichhas an input side connected via the lead 11 to the electrode 10 foracquiring the electrical potential in the heart tissue. This arrangementis simple because only a single electrode 10 is required both forstimulating the heart and for acquiring the reaction thereof Of course,the electrode according to the invention may be used only as stimulationelectrode for stimulating tissue or a measuring electrode for acquiringthe evoked response for e.g. operating in the VDD stimulating mode. Insuch cases either the stimulation generator 120 is programmed not todeliver stimulation pulses or the detector unit 110 not to register anyevoked response (not shown).

The control unit 130 further contains known means for evaluating theelectrical signals received by the detector 110 for making a diagnosisof the condition of the heart depending on e.g. the morphology of theelectrical signal B or how the two electrical signals A and B relate toeach other and/or to the stimulation pulse, and possibly for starting atherapy based on the made diagnosis.

FIG. 4 shows a schematic circuit diagram of a pacemaker in accordancewith a first embodiment of the invention. The stimulation pulsegenerator includes a charge pump 121, a capacitor C₁, e.g. 1 μF, and aswitch SI which, when closed, charges the capacitor to a voltage of e.g.20 V. When the stimulation pulse generator 120 rapidly transfers chargeto the electrode 10, the thickness of the piezoelectric material 50changes and pressure waves are emitted to the heart tissue. It is knownthat mechanical irritation of the endocardium can start a heartcontraction, the mechanical stimulation may decrease the threshold forthe electrical stimulation or may by itself initiate a heartcontraction. Because the piezoelectric electrode 40, 50 functions as acapacitor as well, electrical current is transferred to the tissue whenclosing the switch S₂. Since the capacitance C_(p) of the piezoelectricmaterial preferably is 10 to 100 nF, a relatively high voltage of about5 to 25 volt is needed during a very short time of about 10 to 100 μsfor reaching the stimulation threshold. This voltage may be generatedinductively or capacitively and then be stored on C₁. The voltage ishigher than the voltage at conventional electrodes. The total energyused is, however, about the same as with conventional electrodes sincethe pulse width is small. The detector unit 110 comprising a detectorcircuit 111 and a charge amplifier 112 detects both electric signals Aand B corresponding to the electrical and mechanical evoked responserespectively registered by the piezo electrode 45.

An alternative embodiment of the schematic circuit diagram of FIG. 4 isshown in 20 FIG. 5. In order to influence the stimulation threshold, thestimulation pulse generator 120 may generate a stimulation pulse whichis chopped with a high frequency of e.g. 10 to 100 kHz. The choppedstimulation frequency may be obtained by opening and closing the switchS₂. Due to the chopped stimulation pulse, the piezo sensor generates aseries of pressure waves. Since the high frequency improves theelectrical transmission through the piezo capacitor C_(p) more normalpulse amplitudes may be used.

The piezoelectric electrode 10 may be used together with adefibrillation electrode 300, either as two separated electrodes, i.e.two leads, or in combination on a single lead, whereby the piezoelectricelectrode is placed at the tip of the lead and the intravasculardefibrillation electrode 300 is placed behind the piezo electrode 10 asis shown in FIG. 6.

Thus an electrode for electrically and mechanically stimulating anddetecting evoked response is provided. One skilled in the art willappreciate that the present invention can be practised by other than thedescribed embodiments, which are presented for purposes of illustrationand of limitation, and the present invention is limited only by theclaims which follow.

What is claimed is:
 1. An implantable electrode for an implantablestimulation device, said implantable electrode comprising apiezoelectric electrode having a conductive core covered bybiocompatible piezoelectric material adapted for direct contact withtissue when implanted, said piezoelectric material having apiezoelectric surface, with an entirety of said piezoelectric surfacebeing adapted to participate in electrical and mechanical interactionswith said tissue, said interactions being selected from the groupconsisting of electrically and mechanically transferring stimulationenergy to said tissue, and electrically and mechanically receivingelectrical and mechanical responses of said tissue selected from thegroup consisting of evoked responses and intrinsic responses.
 2. Animplantable electrode as claimed in claim 1 wherein said piezoelectricmaterial has a capacitance in a range between 10 and 100 nF.
 3. Animplantable electrode as claimed in claim 1 wherein said piezoelectricelectrode is a tip electrode.
 4. An implantable electrode as claimed inclaim 3 wherein said piezoelectric electrode is hemispherical.
 5. Animplantable electrode as claimed in claim 3 wherein said piezoelectricelectrode has a planar distal end.
 6. An implantable electrode asclaimed in claim 1 wherein said piezoelectric electrode is a ringelectrode.
 7. An implantable electrode as claimed in claim 6 whereinsaid piezoelectric electrode is coaxial.
 8. An electrode leadconnectable to an implantable stimulation device, said electrode leadcomprising: a piezoelectric electrode having a conductive core coveredby biocompatible piezoelectric material adapted for direct contact withtissue when implanted, said piezoelectric material having apiezoelectric surface, with an entirety of said piezoelectric surfacebeing adapted to participate in electrical and mechanical interactionswith said tissue, said interactions being selected from the groupconsisting of electrically and mechanically transferring stimulationenergy to said tissue, and electrically and mechanically receivingelectrical and mechanical responses of said tissue selected from thegroup consisting of evoked responses and intrinsic responses, and aconductor adapted for delivering stimulation energy to saidpiezoelectric electrode and for conducting electrical signals from saidpiezoelectric electrode representing said response.
 9. An implantablestimulation device for stimulating tissue, comprising: a piezoelectricelectrode having a conductive core covered by biocompatiblepiezoelectric material adapted for direct contact with tissue whenimplanted, said piezoelectric material having a piezoelectric surface,with an entirety of said piezoelectric surface being adapted toparticipate in electrical and mechanical interactions with said tissue,said interactions being selected from the group consisting ofelectrically and mechanically transferring stimulation energy to saidtissue, and electrically and mechanically receiving electrical andmechanical responses of said tissue selected from the group consistingof evoked responses and intrinsic responses, and a conductor adapted fordelivering stimulation energy to said piezoelectric electrode and forconducting electrical signals from said piezoelectric electroderepresenting said response, a stimulation pulse generator connected tosaid conductor adapted for generating said stimulation energy, and adetector unit connected to said conductor adapted for receiving saidsignals representing said responses.
 10. An implantable stimulationdevice as claimed in claim 9 further comprising a control unit connectedto said detector unit, said control unit being supplied with saidsignals received by said detector unit and processing said signals. 11.An implantable stimulation device as claimed in claim 10 wherein saidcontrol unit includes a fusion beat detector stage for detecting fusionbeats in said signals, said fusion beat detector stage determining ifsaid signal arrives before a predetermined time window.
 12. Animplantable stimulation device as claimed in claim 10 wherein saidcontrol unit comprises an analyzing stage for analyzing contractions ofa heart by determining a morphology of said signal.
 13. An implantablestimulation device as claimed in claim 10 comprising a further electrodeconnected to said stimulation pulse generator, and wherein said controlunit is connected to said stimulation pulse generator and includes anA-V timer for operating said stimulation pulse generator for dualchamber pacing using said piezoelectric electrode and said furtherelectrode, said control unit starting said A-V timer after detection ofsaid signal representing a response.
 14. An implantable stimulationdevice as claimed in claim 10 wherein said control unit includes anelectromechanical dissociation detector stage for detectingelectromechanical dissociation by determining whether said signalrepresents both an electrical and a mechanical evoked response of saidtissue after emission of said stimulation energy.
 15. An implantablestimulation device as claimed in claim 9 wherein said stimulation pulsegenerator generates said stimulation energy in a chopped stimulationpulse.
 16. An implantable stimulation device as claimed in claim 15wherein said stimulation pulse generator generates said choppedstimulation pulse with a frequency between 10 and 100 kHz.